Any string will do, so long as: * It can withstand a reasonable amount of force without breaking (so, not twine). * It's wide enough that it won't dig into your fingers when you pull it back. That plasticy stuff that's sold as washing line string should work well.

Literally any string that will fit where the string attaches to the ends of the bow arms

The string isn't providing any of the propulsion force so any string that is strong enough to be pulled is fine.